Recent studies in the industry have shown that the application of a silicon nitride (SIN) thin coating using low temperature PECVD can be very helpful in improving the efficiency of polysilicon (poly-Si) solar cells by reducing reflection from the cells. See, as examples, C. C. Johnson et al., "Plasma-enhanced CVD silicon nitride antireflection coatings for solar cells," Solar Energy, vol. 31, no. 4, pp. 355-358. 1983; P. P. Michiels et al., "Hydrogen passivation of polycrystalline silicon solar cells by plasma deposition of silicon nitride," in Proc. 21st IEEE Photovoltaic Specialist Conf., 1990, pp. 638-643; and R. Kisshore et al., "Growth of silicon nitride films on single and multicrystalline silicon solar cells using PECVD technique," in Proc. 6th Int. Photovotaic and Engineering Conf., (New Delhi, India), 1992, pp. 249-253. The SiN acts as an antireflection coating with a suitable refractive index n, but can also improve the performance of photovoltaic devices by defect/surface passivation. The reason for this feature is that a large amount of atomic hydrogen is produced during the PECVD process. In addition, PECVD is a low-temperature process (about 300.degree. C.), compared to other CVD processes (for instance, LPCVD or APCVD), and has high throughput, good uniformity, better thickness control (&gt;5%), and excellent reproducibility, compared to the physical evaporation techniques. All these advantages make PECVD SiN coating very attractive for solar cells.
The improvements in the minority-carrier diffusion length and quantum efficiency due to a single-layer SiN coating formed by PECVD on poly-Si solar cells have been investigated in the art. See P. P. Michiels et al., "Hydrogen passivation of polycrystalline silicon solar cells by plasma deposition of silicon nitride," in Proc. 21st IEEE Photovotaic Specialist Conf., 1990, pp. 638-643; M. Lemiti et al., "Hydrogenation of multicrystalline silicon from a backside silicon nitride layer," in Proc. 22nd IEEE Photovotaic Specialists Conf., 1991, pp. 1002-1005; J. C. Muller et al., "Improvement of silicon nitride solar cells after thermal processing gettering or passivation?" in Proc. 22nd IEEE Photovotaic Specialists Conf., 1991, pp. 883-886. The beneficial effects of a single-layer PECVD SiN on poly-Si solar cells relative to short-circuit current, open-circuit voltage, and cell efficiency have also been investigated in the industry. In this regard, see S. R. Wenham et al., "Efficiency improvement in screen printed polycrystalline silicon solar cells by plasma treatments," in Proc. 18th IEEE Photovotaic Specialists Conf., 1985, pp. 1008-1013; R. Kisshore et al., "Growth of silicon nitride films on single and multicrystalline silicon solar cells using PECVD technique," in Proc. 6th Int. Photovotaic and Engineering Conf., (New Delhi, India), pp. 249-253, 1992; and S. Narayanan et al., "Silicon nitride AR coating for low cost silicon solar cells," in Proc. 6th Int. Photovoltaic and Engineering Conf. (New Delhi, India), pp. 133-136, 1992.
A double-layer MgF.sub.2 /ZnS antireflection coating is also known in the art and is often used in high-efficiency single-crystal silicon cells fabricated in the laboratory because the MgF.sub.2 /ZnS coating has much better antireflection properties than that of a single-layer SiN coating. In this regard, see M. A. Green et al., "High efficiency silicon solar cells," IEEE Trans. Electron Devices, vol, ED-31, pp. 697-683, 1984. MgF.sub.2 has a refractive index n of 1.35, and ZnS has an index n of 2.4. The refractive index n of stoichiometric silicon nitride (Si.sub.3 N.sub.4) is around 2.0. Therefore, it is not as suitable as a double-layer antireflection coating as ZnS. On the other hand, SiN applied by PECVD is an amorphous form of Si.sub.x N.sub.y :H.sub.z, in which x, y, and z strongly depend on the deposition conditions, especially on the gas flow ratio of NH.sub.3 /SiH.sub.4 and substrate temperature. The refractive index n of SiN depends on the ratio of N/Si in the coating and can be controlled in the range of 1.96-3.5 by adjusting the ratio of NH.sub.3 /SiH.sub.4. Thus, the PECVD process provides an opportunity for obtaining higher index SiN coating by making it silicon-rich. However, silicon-rich coatings generally show high absorption, and therefore absorption losses must be considered when high index SiN coatings are used.
Although use of either the single-layer SiN coating or the double-layer MgF.sub.2 /ZnS coating as an antireflection mechanism has merit, the formation of these coatings has significant disadvantages. The single-layer SiN is not as effective in antireflection as the double-layer MgF.sub.2 /ZnS coating, and suffers significant reflection loss. Moreover, the double-layer MgF.sub.2 /ZnS coating is formed by thermal evaporation, which is extremely time consuming. The formation requires undesirably low vacuum conditions, generally around 10.sup.-6 torr. Hence, production of these coatings on a mass commercial scale is expensive.